Proteinase K (EC 3.4.21.64) is a serine protease which can be isolated from the fungus Tritirachium album limber (Ebeling, W., et al., Eur. J. Biochem. 47 (1974) 91-97, and German Patent DE 19 65 281). Proteinase K is capable of digesting native keratin and remains active in the presence of low concentrations of SDS (sodium dodecyl sulfate). Therefore, proteinase K is very useful for the isolation of native RNAs in high yields (Wiegers, U., and Hilz, H., Biochem. Biophys. Res. Commun. 44 (1971) 513-519).
Proteinase K is also widely used in the field of PCR (polymerase chain reaction). In such methods there are used, in intermediate steps, certain enzymes such as restriction endonucleases, polymerases, and DNA modifying enzymes. It is often necessary to inactivate these enzymes before performing the next PCR step. Such inactivation is preferably performed by the use of proteinase K. It is therefore necessary that proteinase K is of high purity and is especially not contaminated with deoxyribonuclease, endonucleases, exonucleases, and DNA.